FACULTY OF SCIENCE AND TECHNOLOGY
DEPARTMENT OF BIOLOGICAL SCEINCE
HBIO 102 /DIVERSITY OF LIFE 2
QUESTION: Outline the characteristics of classes in the phylum Echinodermata
DUE DATE: 27 September 2013
This large phylum of marine organism are well preserved in the fossils record and are represented by the above five distinct classes. Many adult echinoderms are recognized by their 5-part (pentamerous) radial symmetry. The radial symmetry of the adult echinoderms is thought to have been added later in the evolutionary history of this group, as a modification for slow moving life on the ocean floor. This type of specialization is called secondary radial symmetry. Echinoderms are deuterostomes coelomates. In deuterostomes, coelom formation occurs by an out pocketing (rather than a splitting) of the mesodermal fining in the embryo, the anus is formed from the blastopore while the mouth is formed from a second embryonic opening.(Campbell,2011)
Another characteristic typical of the echinoderms is a mesodermally derived endoskeleton. It is made up of many small calcareous plates, which may be separate (as in the sea cucumbers) or fused into a rigid framework or test (as in the sea urchins and sand dollar). In many echinoderms, the skeletal plates bear protruding spines. Perhaps the most unique characteristic of echinoderms is the water vascular system consisting of numerous water filled tubes ending in a large number of tube feet. The water vascular system is formed from the coelom during embryonic development and is important in locomotion, food handling and attachment.(Neilson , 1995) Echinoderms are among the most distinctive of all animal phyla. Inclusion in the phylum is readily diagnosable on the basis of the four synapomorphies below. Most of these features are present, or can be inferred, even in the earliest fossils. Together these synapomorphies define much of what makes the functional biology of echinoderms distinctive from that of other metazoans.
Calcitic skeleton composed of many ossicles: The bio mineral matrix of echinoderm skeleton is composed of calcium carbonate and several proteins. The calcite is deposited as numerous tiny crystals, but all them lay on the same axis within an ossicle. For this reason ossicles are birefringent under polarizing light. Ossicles are not solid, but have a sponge like microstructure called stereom that is unique to the phylum. Embryologically,echinoderms ossicles are a true endoskeleton, since they are produced by mesenchymal cells and are usually covered by the epidermis. Functionally, however, the majority of ossicles act more like an exoskeleton, lying just under the epidermis and enclosing most other tissue in the flexible but tough covering. (Pearse , 1994)
Water vascular system: The water vascular system performs many important functions in echinoderms, including locomotion, respiration, and feeding; in addition, most sensory neurons are located at the termini of podia(tube feet)which are part of this organ system. The water vascular system may have evolved from simple tentacular system similar to those in other deuterostomes phyla, such as the tentacles of pterobranch hemichordates. However, there are many derived features of the water vascular system in echinoderms, including: an embryological origin from left mesocoel, podia arranged along branches (ambulacra), and a central circumesophageal ring.(Pearse,1994)
Mutable collagenous tissue: The ossicles of echinoderms are connected by ligaments composed predominantly of collagen. The material properties of this connective tissue are mutable on short timescales, under neuronal control. Ligaments are normally “locked” (rigid), but can be temporarily “unlocked” (loosened). This provides some...
References: Campbell, N and Reece, J. (2011). Biology (6th Ed), Benjamin Cummings
Neilson, C. (1995). Animal Evolution: Interrelationships of living phyla. Oxford University press, Oxford
Pearse, V.B., and J.S, Pearse.1994.Echinoderm phylogeny and the place of the concentricyloids, pp 121-126 in Echinoderms through time (B.David,A. Guile, J, P.Feral, and M. Roux, eds). Balkema, Rotterdam.
Smith, A.B. 1984. Classification of the Echinodermata.Palaeontology 27:431-459
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